(MsgpackHandle).WriteExt

Description

WriteExt flag supports encoding configured extensions with extension tags.
It also controls whether other elements of the new spec are encoded (ie Str8).

With WriteExt=false, configured extensions are serialized as raw bytes
and Str8 is not encoded.

A stream can still be decoded into a typed value, provided an appropriate value
is provided, but the type cannot be inferred from the stream. If no appropriate
type is provided (e.g. decoding into a nil interface{}), you get back
a []byte or string based on the setting of RawToString.